Rivers in urban polder areas generally have poor natural fluidity and low self-purification capacity, so the water environment deterioration is a common problem in these areas. Dongting Street, belonging to Wuxi, is a diked area in the Yundong Polder. It has the general characteristics of the urban polder areas. Dongting Street has a large amount of domestic sewage discharge, suggesting the main pollution factor is ammonia nitrogen. This research uses the MIKE 11 model intended to control the concentration of ammonia nitrogen. By using the MIKE 11 model, this research analyzes the reduction rate of the concentration of ammonia nitrogen under the condition that one or more of the three factors improved. Results suggest that the peripheral water quality has the greatest impact on the water environment at the studied area. When the peripheral water quality was improved to class Ⅳ surface water standard of China (GB3838-2002), the reduction rate of ammonia nitrogen was 67.3%. The reduction rate was 53.6% when the pollution level increased to 98%. At last, when the Ⅳ water source (6 m3/s) was input through gate and pump station, the reduction rate was 50.3%. When two or more factors were simultaneously improved, there was a superposition effect on the improvement of water environment, and the ammonia nitrogen concentration reduced by about 85.5% at most.